Title:
Confirmation of One of the Coldest Known Brown Dwarfs

Abstract: Using two epochs of 4.5um images from the Infrared Array Camera (IRAC) on
board the Spitzer Space Telescope, we recently identified a common proper
motion companion to the white dwarf WD 0806-661 that is a candidate for the
coldest known brown dwarf. To verify its cool nature, we have obtained images
of this object at 3.6um with IRAC, at J with HAWK-I on the Very Large
Telescope, and in a filter covering the red half of J with FourStar on
Magellan. WD 0806-661 B is detected by IRAC but not HAWK-I or FourStar. From
these data we measure colors of [3.6]-[4.5]=2.77+/-0.16 and J-[4.5]>7.0
(SNR<3). Based on these colors and its absolute magnitudes, WD 0806-661 B is
the coldest companion directly imaged outside of the solar system and is a
contender for the coldest known brown dwarf with the Y dwarf WISEP J1828+2650.
It is unclear which of these two objects is colder given the available data. A
comparison of its absolute magnitude at 4.5um to the predictions of theoretical
spectra and evolutionary models suggests that WD 0806-661 B has T=300-345 K.